Heat Transfer: Radiation

Concept

Thermal radiation is the transfer of heat via electromagnetic waves emitted by matter due to its temperature. Unlike conduction and convection, radiation requires no intervening medium and can travel perfectly through a vacuum (e.g., solar radiation reaching Earth). All bodies at a temperature above absolute zero emit thermal radiation. In engineering systems, radiation becomes the dominant mode of heat transfer at high temperatures.

Formula & Method

The maximum possible radiation emitted by an ideal surface (a blackbody) is given by the Stefan-Boltzmann law: $q_{max} = \sigma A T^4$ Where:

• $\sigma$ = Stefan-Boltzmann constant ($5.67 \times 10^{-8} \text{ W/(m}^2 \cdot \text{K}^4)$)
• $A$ = Surface area
• $T$ = Absolute temperature (in Kelvin)

Real surfaces emit less radiation than a blackbody, accounted for by the emissivity ($\epsilon$, ranging from 0 to 1): $q_{emit} = \epsilon \sigma A T^4$ The net radiative heat exchange between two surfaces also depends on their geometric orientation to each other, defined by a View Factor ($F_{12}$).

Engineering Application

Because radiation is proportional to temperature to the fourth power ($T^4$), it dominates energy loss in high-temperature applications like industrial furnaces, boilers, and spacecraft thermal control. Engineers manipulate surface emissivity to control radiation; for instance, using highly polished, low-emissivity metals for insulation (thermos flasks or satellite shielding) or high-emissivity matte black surfaces for heat dissipation (heat sinks).